Sk2x/Sk4x: add negate(), operator -()

BUG=skia:

Review URL: https://codereview.chromium.org/1027783002
diff --git a/tests/Sk2xTest.cpp b/tests/Sk2xTest.cpp
index 4cecc07..a3b86b2 100644
--- a/tests/Sk2xTest.cpp
+++ b/tests/Sk2xTest.cpp
@@ -51,6 +51,8 @@
     REPORTER_ASSERT(r, eq(Sk2x<T>::Min(a, d), 2, 4));
     REPORTER_ASSERT(r, eq(Sk2x<T>::Max(a, d), 4, 5));
 
+    REPORTER_ASSERT(r, eq(-d, -2, -5));
+
     // A bit of both.
     a += d;
     a *= d;